How to Model Smooth Models for Games with Low Poly
I've worked with various games engines for about 18 years now so I'll try my best to answer your questions.

For something kinda small like that I'd say 200 tri's at the very most. 2-5k is a LOT for a cup considering that weapons tend to be about 2k max. Also creating good levels of detial (LODs) might help a lot as well but I've not really worked that much with UE4 so not sure if people even do that for this engine.

I suggest looking into something called retopology. You basically take your high poly model and then draw your low poly over it making sure to have good topology. It's not super important for non-animated/rigged things but it's good practice. Here's a nice tutorial that demonstrates how to do it and the add-on is free. It'll help you greatly! What software are you using for your texture work? looking into texture baking would benefit you greatly as well, AFTER watching this video.
Blender 2.9 Retopology Getting Started

You won't have to remake textures and normal maps for the LOD's as the LOD's use the texture information of the model itself and, let's face it, from a distance you can't really see normal maps anyway.


If you need further help just ask ^.^